31.2 Leakages from off-channel water storages
The volume reported (930 ML) represents leakage during the 2011–12 year from off-channel water storages to both groundwater and the landscape as it was not possible to separately quantify each of these items.
Bureau of Meteorology (the Bureau): National Climate Centre daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; South Australian Department of Environment, Water and Natural Resources (DEWNR): Geographical Information System layers; Geoscience Australia: 9 arc-second digital elevation model (DEM).
The Spatial Tool for Estimating Dam Impacts (STEDI) model (Sinclair Knight Merz 2010) was used to determine the volume of water leaking from off-channel water storages. Data input to the model included gridded climate datasets; runoff from the AWRA-L version 2.0.0 (Van Dijk 2010) model; and dam details derived from spatial data (surface area, volume, location and catchment area). Model parameters included a leakage factor of 5% per year.
The Adelaide region has more than 10,000 off-channel water storages. This is the maximum number of off-channel water storages that the STEDI model can handle for a single region. Therefore, the Adelaide region was split into two smaller subregions. The region was divided using the boundaries of the AHGF contracted catchments between McLaren Vale and the Onkaparinga Valley. The northern region includes the Barossa Valley, the Northern Adelaide Plains, and the River Torrens, Patawalonga and Onkaparinga catchments. The southern region includes catchments throughout McLaren Vale and the Fleurieu Peninsula.
Only off-channel water storages filled primarily by rainfall-runoff were considered. These were determined from waterbody mapping provided by the South Australian Department for Water, and excluded waterbodies that were within the Virginia Pipeline Scheme service area and waterbodies that were less than 20 metres away from a channel of second order or higher, or an active bore. A 9 arc-second DEM was used to determine the catchment area of each off-channel water storage in the Adelaide region.
The STEDI model determines the water stored in each off-channel water storage at each time step. STEDI assumes that water will leak from the storage unless it empties, at which time leakage will cease.
Assumptions, limitations, caveats and approximations
- The gridded climate input data were subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).
- The estimated volume available in storage for evaporation was subject to the assumptions associated with the STEDI model and the parameters used.
- The spatial extent of water bodies was subject to the assumptions and methods associated with the spatial data provided by the DEWNR.
The uncertainty estimate is not quantified.